Currently, the machines used for machining large crankshafts comprise a machining tool with a displacement running the length of the crankshaft undergoing machining. Likewise, the machine comprises two end supporting elements, usually called the headstock and the tailstock, which hold the crankshaft at its ends and which also exert torque oriented according to a central axis of the crankshaft, transmitting thereto a rotational movement on its axis that allows the machining tool to machine the whole outer surface of the crankshaft, by means of parallel movement to said central axis or trunnion, in accordance with the same operational principle as a lathe.
This equipment and machines for the machining of large crankshafts allow the whole crankshaft to be machined, with the exception of the ends, given that said ends correspond to the areas where the crankshaft is anchored to the plates of the end supporting elements of the machine. Normally, said ends of the crankshaft are machined at a later stage, using for this equipment other than crankshaft machining equipment, and for this reason the crankshaft is always supported by end supporting elements during its machining.
With this kind of large crankshafts, during the machining of the crankpins, as a result of the fact that the crankpin shafts are not contained on the central axis of the crankshaft whereby it is anchored to the end supporting elements of the machine, a great deal of shear stress is produced in the crankshaft, which, because it is not a rigid piece, result in excessive distortion of the crankshaft, which, in many cases, prevents suitable machining tolerance from being reached, producing vibrations in the crankshaft that hinder the machining process, for example roughing, in optimum conditions for achieving a low cycle time.
With the aim of resolving the abovementioned drawback, the machines for machining this kind of crankshaft include a central or intermediate supporting element, commonly called a motorised steady rest, which allows the crankshaft to be supported in the centre and exert, like the headstock and tailstock, torque oriented according to the central axis of the crankshaft, transmitting thereto a rotational movement on its own axis.
In order to machine the whole surface of the crankshaft, said intermediate supporting element must be able to change position, changing its anchoring point to the crankshaft by shifting from one supporting element to another and allowing the machine to machine the supporting element whereby the intermediate supporting element was anchored, thereby machining the whole crankshaft.
Likewise, there are machines that comprise other types of intermediate supporting elements that do not transmit rotation to the crankshaft, thereby serving to avoid excessive distortion to the crankshaft during its machining as a result of the action of its own weight or the stress that is produced during said machining operations. These steady rests allow machining to be carried out with greater precision, but have the drawback that they constitute a reaction that results in an application of additional torsion actions on the crankshaft. Bearing this negative side effect in mind, a determining factor in avoiding excessive stress on the crankshaft, especially at moments of excessive torsion, is that the crankshaft's rotation should be perfectly synchronised, both at the ends, i.e. the headstock and the tailstock, and in the areas of the intermediate steady rests.
Usually, these machines consist of a headstock, a tailstock and a motorised steady rest in an intermediate position, so that these three supporting points hold and transmit rotational movement to the crankshaft. In view of this embodiment, it can be clearly seen here that the rotational movements transmitted by each of these three elements can generate torsion stress in the crankshaft that may result in excessive distortion thereof if they are not perfectly synchronised. For this reason, in order for machining to be achieved properly on the crankshaft, it is important that the rotation of these three elements are synchronised.
Currently, there are means of synchronising rotational movement at the end supporting elements and the motorised steady rest of these machines for machine crankshafts. Said means of synchronisation consists of a set bar located between the headstock and the tailstock that allow the rotational movement of the headstock to be linked to the rotational movement of the tailstock, for which purpose the bar is of an equivalent length to the distance between end supporting elements of the machine.
In turn, to connect the set bar to the motorised steady rest there is a clutch mechanism that allows the bar to be engaged with the steady rest for the transmission of movement, so that the rotation of said mechanism, and therefore the steady rest's rotation, and the rotation of both the headstock and the tailstock are in synchrony with the rotation of the bar. On the other hand, the clutch mechanism allows the set bar to be disengaged when the position of the motorised steady rest has to be changed, as this rest is not in a fixed position in relation to the crankshaft throughout the machining process, as explained above.
In this way, by joining the motorised steady rest to the bar, the rotation motor of the steady rest is built into to the rotation motor of the headstock, and therefore the result is the same as having two motors connected together in series to rotate the crankshaft. If the bar has two motors in series, one located in the headstock and the other in the tailstock, the effect is equivalent to having a third motor in series, when the bar is engaging with the motorised steady rest.
The main drawback of this synchronisation system is that, given the great length of the crankshaft, it is difficult to perfectly synchronize the rotation of the end supporting elements, generating excessive torsion stress on the crankshaft, in addition to it being a mechanical synchronisation system, excessively complicating the manufacture and operation of the machine, being very unreliable and considerably increasing its complexity. Likewise, these synchronisation means has serious operational disadvantages resulting from the fact that the bar has to be engageable and disengageable to the motorised steady rest in order to allow said rest to move along the whole crankshaft in different supporting elements.